Race condition: ModbusRTUServer.begin() sometimes quietly rejects config #135

I was experiencing a very difficult to debug issue with this library on my RP2040 wherein seemingly very similar code produced a different result. Ultimately this proved to be a race condition, in which ModbusRTUServer.begin() sometimes was not accurately loading in my provided serial format and instead used the default.

int ModbusRTUServerClass::begin(int id, unsigned long baudrate, uint16_t config)

Symptoms:

• Device ignoring provided argument provided as argument config above, instead just using the default, SERIAL_8N1

Temporary resolution to issue:

• Call ModbusRTUServerClass.begin() multiple times (I put mine in a for loop of 100 times). This is an extremely hacky solution and not guaranteed to work 100% of times, but it proves the issue exists in the wild.

Proposed resolution:

• Root cause the race condition / issue with successfully setting this config in all situations, and add functionality to check that the config has successfully been loaded, possibly with a set + get combo with a delay for the loading to take effect.

Details:

• RP2040
• Latest ArduinoModbus release
• Easiest to replicate at high baud rates (115200 and above)